Current semiconductor processing generally uses an elaborate system of metallization interconnects to couple the various devices which have been fabricated on the semiconductor substrate. Commonly, aluminum or some other metal is deposited and then patterned to form interconnect paths along the surface of the silicon substrate. In most processes, a dielectric or insulated layer is then deposited over this first metal layer. Via openings are etched through the dielectric layer and a second metal layer is then deposited. The second metal layer covers the dielectric layer and fills the vias to make electrical contact with the underlying first metal layer. The purpose of the dielectric layer is to provide an insulator between the metal layers. Thus, if a third metal layer is required, then second dielectric layer must be deposited over the second metal layer to provide electrical insulation.
Often, the different deposited dielectric layers are conformal layers which correspond in height to the underlying metal lines. Thus, the upper surface of each dielectric layer is characterized by a series of nonplanar height variations which has been found to be undesirable. The art provides various methods for planarizing the surface of the dielectric layers. One such method employs abrasive polishing to remove the protruding steps along the surface of the dielectric layer. In this method, the silicon substrate is placed faced down on a table covered with a polishing pad which has been coated with a slurry or abrasive material. Both the substrate and the table are then rotated relative to each other to remove the protrusions on the substrate. This process of planarizing the substrate surface is generally referred to as chemical mechanical polishing (CMP).
One factor in achieving and maintaining a high and stable polishing rate for CMP is pad conditioning which is a technique where the pad surface is prepared into a proper state for subsequent polishing work. Various methods are available to condition the polishing pad. One method cuts circumferential grooves into the polishing pad surface to channel slurry between the substrate surface and the pad. These grooves are formed prior to polishing by means of a milling machine, a lathe, or a press. A problem with this method is that the ridges forming the grooves become worn down after repeated polishing cycles. The smoothed out polishing surface results in a reduction of slurry delivery beneath the substrate surface. This degradation in pad roughness over time results in low, unstable, and unpredictable polish rates.
A related method simply provides a polishing pad with preformed, circumferential, triangular grooves. Then during the polishing step, a diamond tip on an oscillating stylus cuts additional radial grooves into the polishing pad to further channel the slurry between the pad and the substrate surface. This method suffers similar drawbacks as the previous method because the circumferential grooves eventually get worn away, and the radial grooves being cut into the pad during polishing decrease the life of the pad.
A third alternative method uses a diamond conditioning disk to condition the polishing pad prior to polishing of the wafer. This method suffers the drawback of the diamonds eventually being lost from the conditioning disk and becoming mixed in with the slurry. The loose diamond granules in the slurry can scratch the polished surface of the wafer. Also, the diamond conditioning disk must eventually be discarded once the diamonds are lost from the surface. Another disadvantage is that this method also abrades the surface of the polishing pad by removing a layer of the polishing pad surface.
All of these above methods are known methods for polishing of dielectric layers. A different challenge exists when the metal layers need polishing. A metal layer requires polishing, for example, when a layer is deposited for filling vias. Once the vias are filled, then the excess metal is polished away before the next device fabrication processing step. The above methods do not work for metal polishing. One reason is that all of them abrade the polishing pad in a manner inconsistent with the requirements of metal polishing. The pad surface does not yield uniform polished surfaces without continual reconditioning of the polish pad. Moreover, abrading the polishing surface dramatically reduces the useful life of the pad, and it must be changed out often, causing delay in the polishing cycle time as well as adding to the cost of the process. Additionally, the diamond disk method introduces corrosion problems due to the metal on the substrate reacting with the polishing slurry due to the diamonds in the slurry.
One method evaluated for metal polishing involved using a brush to roughen the surface of the polishing pad as the preconditioning step. This method had several drawbacks. First, residue accumulated in the bristles making the brushes hard to clean. Secondly, the bristles on the brushes tended to break off in the process, leading to undesirable matter in the slurry. Furthermore, the brushes wore out quickly even when the bristles did not break, making this solution very impractical.
One method for successfully polishing metal is to use dummy or blanket wafers as a preconditioning step for the polishing pad. This method uses blank silicon wafers having a top surface layer of tungsten in the polishing apparatus to prepare the surface of the polishing pad for subsequent real polishing work. The polishing rate and uniformity of the polished surface are monitored after each blanket wafer until a stable polishing rate is achieved. Only then are real wafers needing to be polished placed on the polishing apparatus to be processed. This method is time consuming because approximately 10 blanket wafers are required before the polishing pad surface is ready for metal polishing. This preconditioning step requires approximately 40-50 minutes to complete and must be repeated whenever a new polishing pad is installed and also whenever the polishing pad has remained idle for a short period of time, approximately 15 minutes. Thus, a method for quickly attaining a saturated and stable surface for polishing to avoid pockets of slurry giving rise to an uneven polished surface is desired.